Patch Preview Link for bug where references to other entities being previewed don't display

[stephen-cox]

What does this change?

Patched Preview Link for issue #771, drupal.org issue: https://www.drupal.org/project/preview_link/issues/3481523

The patch changes the node access hook so Preview Links access handler is run for all pages included in a preview link entity. Without the hook it only runs for the page being viewed.

How to test

See #771 (comment) on how to reproduce the issue this should fix.

How can we measure success?

Previewing more than one page becomes easier and works as people expect.

Have we considered potential risks?

This changes an access handler so has the potential to allow more than it should.

[stephen-cox]

Tests failures are fixed with #787

[ekes]

To clarify if I understand what your patch is doing:
Previously the access check override looked at the route, checked if it is in preview mode, checked if the route entity is one that can be previewed.
Now the access check, looks if it's in preview mode, runs through all entities that can be previewed, and sees if there's a match?

[ekes]

Hit wrong button, was just the question :-)

[stephen-cox]

Previously the access check override looked at the route, checked if it is in preview mode, checked if the route entity is one that can be previewed.
Now the access check, looks if it's in preview mode, runs through all entities that can be previewed, and sees if there's a match?

That's right - previously is only checked access for the entity being previewed but now checks whether the entity is included in a related preview link.

[finnlewis]

@ekes is happy with this, let's merge it!